I can't tell you how many times I've struggled with a problem at the end of the day only to solve it within 5-10 minutes the next morning. I don't know why that's the case, but it works. It could be the "sleep on it" method where our brains continue churning on the problem. It could be just walking away and coming back to gain a new perspective.
Now that I have this knowledge, I try and stay sensitive to it. If I'm ever in the position of trying to push through solving a problem at the end of the day, I'm quicker to call it a day and tackle it again in the morning.
Sometimes tomorrow is better than today.